Method for forming a three-dimensional article by fusion of a powdered medium in a powder bed

ABSTRACT

Methods and apparatus for forming a three-dimensional article in which a powdered medium is distributed in a powder bed, heated to a temperature below its melting point, and fused by means of a laser beam projected in the form of an image of a cross section of the three-dimensional article. The image of the cross-sectional layer is created by spatial light modulation of the laser beam using an LCoS display. During the fabrication process, characteristics of the image of the cross-sectional layer are controlled through monitoring of the projection of a holographic representation of the image using one or more imaging devices.

RELATED APPLICATIONS

This application is a Divisional application of U.S. application Ser.No. 15/914,741, filed on 7 Mar. 2018, which is a non-provisional patentapplication of and claims priority to U.S. Provisional Application No.62/474,836, filed 22 Mar. 2017, each of which is incorporated byreference herein.

FIELD OF THE INVENTION

The present invention relates generally to methods and apparatus foradditive manufacturing, and in particular to such methods and apparatusas facilitate formation of entire cross-sectional layers of an object ata time.

BACKGROUND

So-called “3D printing” or, more generally, additive manufacturing, is abroad term used to describe processes to fabricate three-dimensionalobjects from digital data files under computer control. A number ofdifferent additive manufacturing techniques have been developed,including selective laser sintering (SLS). SLS involves the fusing ofmaterial, typically a metal, polymer, or ceramic powder, at points inspace defined by a digital model file, using a laser. For a givencross-sectional layer of the model, the focal point of the laser isscanned over a bed of powdered material, causing the material to form asolid mass at the points heated, individually, by the laser. After eachcross-section is scanned, the powder bed is lowered, a new layer of thematerial is applied, and the process is repeated. This continues,point-by-point for each cross-sectional layer of the object underfabrication until the desired object is completed.

SLS is a popular method for producing prototypes of new designs and forfabricating components. However, because conventional SLS printers relyon a single laser beam scanned over what may be a large cross-section ofa desired object, printing times can be quite long. These long printingtimes can act as a bottleneck to rapid prototype development and othermanufacturing tasks.

SUMMARY OF THE INVENTION

Described herein are methods and apparatus for forming athree-dimensional article by fusion of a powdered medium in a powderbed. In one embodiment, a first layer of a powdered medium (e.g., ametal, polyamide, or other material) is distributed over a powder bed.This may be accomplished by rolling the material in a thin layer overthe powder bed, or by otherwise depositing the layer over same, so thata relatively thin, uniform layer of the powdered medium is distributedthereon. In some instances, the powdered medium may be distributedthrough gravity feed and then rolled or scraped to form the relativelythin, uniform layer on the powder bed. Once distributed, the relativelythin, uniform layer of powdered medium, or at least a portion thereof,may be heated to a temperature below its melting point. This heating maybe accomplished in any of a variety of ways, including using infra-redheaters, and/or by directing a laser beam towards the powder bed througha defined optical path. Where a laser is used, the beam is preferablymodified to have desired energy over its beam width and is directed toencompass a working space of the powder bed, or at least a portionthereof.

An image of a cross-sectional layer of an object (complete or at leastpartially complete) to be fabricated is focused on the layer of thepowdered medium distributed over said powder bed, or at least a portionthereof. The image of the cross-sectional layer is preferably created byspatial light modulation of a laser beam and projection of a resultingspatial light modulated laser beam through a defined optical path ontothe layer of the powdered medium (or portion thereof). The spatial lightmodulated laser beam has sufficient energy to fuse that portion of thepowdered medium in locations in the powder bed corresponding to theimage of the cross-sectional layer of the object to be fabricated toform an integral layer of the powdered medium having a shapecorresponding to that image of the cross-sectional layer of the object.Other portions of the powdered medium on the powder bed remain unfusedand surround the integral or fused portion of the layer of powderedmedium, thereby supporting it.

Next, a second layer of the powdered medium is distributed over thefirst layer, and the forgoing process is repeated using an image of asecond cross-sectional layer of the object to be fabricated to form anintegral layer of the powdered medium having a shape correspondingthereto. This process is repeated for additional layers of the powderedmedium, each being distributed over immediately preceding layersthereof, and additional images of respective additional cross-sectionallayers of the object, so as to form the three-dimensional article.

During the above-described fabrication process, the image quality of therespective images of the cross-sectional layers of the object at thepowder bed may be controlled through monitoring using one or moreimaging devices, e.g., infra-red imaging devices and/or charge coupleddevice (CCD) cameras. By evaluating images of the layers as they arebeing fused, the laser light incident on the respective layers of thepowdered medium may be controlled in accordance therewith. For example,an image of a cross-sectional layer of the three-dimensional articleused to produce the article under fabrication may be modified accordingto feedback provided by one or more imaging devices so that gray levelsof one or more areas of the image are adjusted from those associatedwith an original version of image. The feedback would include imageanalysis of a projection of the original image of the respectivecross-sectional layer on the powdered medium in the powder bed. Theimaging devices may provide images in the infra-red and/or visibleportions of the spectrum.

In the above-described process, where a laser is used to heat thoseportions of a layer of the powdered medium to a temperature below itsmelting point, that laser and the laser used to form the spatial lightmodulated laser beam may be created from a common laser beam using abeam splitter.

In the above-described process, spatial light modulation of the laser toform the spatial light modulated laser beam may be accomplished bygenerating a holographic image of a respective cross-sectional layer ofthe object on a liquid crystal on silicon (LCoS) panel, directing anon-spatial light modulated portion of the laser beam to be incident onthe holographic image, and using a reflected portion of the laser beamfrom the holographic image of the cross-sectional layer of the object onthe LCoS panel as the spatial light modulated first laser beam. In suchinstances, the holographic image of the cross-sectional layer of theobject on the LCoS panel maybe corrected for image distortions, e.g.,0th order diffraction effects in the spatial light modulated first laserbeam at an image plane and other distortions.

These and further aspects of the present invention are described ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and notlimitation, in the figures of the accompanying drawings, in which:

FIGS. 1A-1C illustrate an embodiment of a system for powder bed fusionconfigured in accordance with embodiments of the present invention;

FIG. 2 illustrates an example of an imaging system for use in connectionwith the system for powder bed fusion shown in FIGS. 1A-1C;

FIGS. 3A-3C illustrate various examples of projecting optics for use inconnection with the imaging system shown in FIGS. 2, 4, and 6; inparticular, FIG. 3A illustrates projecting optics with a focusing lensrealizing Fourier transformation and a focusing lens realizingprojection; FIG. 3B illustrates projecting optics with a focusing lensrealizing Fourier transformation and an objective realizing projection;and FIG. 3C illustrates projecting optics with a focusing lens realizingFourier transformation and a negative lens realizing projection;

FIG. 4 illustrates another example of an imaging system for use inconnection with the system for powder bed fusion shown in FIGS. 1A-1C;in this example, laser light is not collimated, and a collimation lensis used for both collimation and beam expansion;

FIG. 5 illustrates another example of an imaging system for use inconnection with the system for powder bed fusion shown in FIGS. 1A-1C;in this example, laser light is collimated, and a polarizer anddivergent lens realize projection and direct laser light to a spatiallight modulator;

FIG. 6 illustrates yet another example of an imaging system for use inconnection with the system for powder bed fusion shown in FIGS. 1A-1C;in this example, a second optical path utilizes a mirror galvanometersystem for high definition edges;

FIG. 7 illustrates an example of computer-generated holographic imageand resulting far field image produced by imaging systems for use inconnection with the system for powder bed fusion shown in FIGS. 1A-1C;and

FIGS. 8 and 9 illustrate examples of adjusting a holographic image of across-section of an object to be fabricated with a system for powder bedfusion in response to feedback provided by an infra-red monitoringsystem configured in accordance with embodiments of the presentinvention.

DETAILED DESCRIPTION

Described herein are methods and apparatus for forming athree-dimensional article by fusion of a powdered medium in a powderbed. As shown in FIGS. 1A-1C, a first layer of a powdered medium 10(e.g., a metal, polyamide, or other material) is distributed over apowder bed 12. This may be accomplished by spreading the material in athin layer over the powder bed using a roller motion mechanism 14, or byotherwise depositing the layer over same, so that a relatively thin,uniform layer of the powdered medium is distributed on the powder bed.In some instances, the powdered medium may be distributed throughgravity feed and then rolled or scraped to form the relatively thin,uniform layer on the powder bed.

Once distributed, the relatively thin, uniform layer of powdered medium,or at least a portion thereof in a work area 16, may be heated to atemperature below its melting point. This heating may be accomplished inany of a variety of ways, including using infra-red lamps, and/or bydirecting a laser beam having a desired energy distribution over itsbeam width towards the powder bed through a defined optical path asdiscussed further below. Where a laser is used, the beam width of thelaser is preferably wide enough to encompass the work area 16 of thepowder bed 12, or at least a portion thereof.

An image of a cross-sectional layer of a work piece (i.e., an object tobe fabricated) is focused on the layer of the powdered mediumdistributed over the work area 16 of powder bed 12, using a laser source18 and imaging system 20. As discussed further below, the image of thefirst cross-sectional layer (e.g., a complete cross-section or at leasta partially complete cross-section) is preferably created by spatiallight modulation of a laser beam 22 and projection of a resultingspatial light modulated laser beam 24 through a defined optical pathonto the layer of the powdered medium in the work area 16. The spatiallight modulated laser beam has sufficient energy to fuse that portion ofthe powdered medium in locations in the work area of the powder bedcorresponding to the image of the first cross-sectional layer of theobject to be fabricated to form an integral layer of the powdered mediumhaving a shape corresponding to that image of the first cross-sectionallayer of the object. Other portions of the powdered medium on the powderbed remain unfused and surround the integral or fused portion of thelayer of powdered medium, supporting it.

Next, as shown in FIG. 1C, the powder bed 12 is lowered, and a secondlayer of the powdered medium 10 is distributed over the first layer(e.g., using roller 14), and the foregoing process is repeated using animage of a second cross-sectional layer of the object to be fabricated,to form an integral layer of the powdered medium having a shapecorresponding thereto. This process may be repeated for additionallayers of the powdered medium, each being distributed over immediatelypreceding layers thereof, and additional images of respective additionalcross-sectional layer of the object, so as to form the three-dimensionalarticle.

During the above-described fabrication process, the image quality of therespective images of the cross-sectional layers of the object at thepowder bed may be controlled through monitoring using one or moreimaging devices 26 (shown in FIG. 1C only, for clarity). The imagingdevices may provide images in the infra-red and/or visible portion ofthe spectrum and may be infra-red imaging devices, CCDs, and/or otherimaging devices. By evaluating the images of the layers as they arebeing fused, e.g., using imaging device 26 and controller 28, times ofprojecting of the laser on the respective layers of the powdered mediumin the powder bed and/or intensity of the laser beam (or, moregenerally, times of the fusing process) may be set and adjusted inaccordance therewith. Controller 28 may, for example, be acomputer-based controller configured (under appropriate softwareinstructions) to receive infra-red images from imaging device 26 and toevaluate said images (e.g., based on color or other characteristics) todetermine properties of each layer as it is created. For example, pixelsof the infra-red images provided by imaging device 26 may be assessedagainst cross sectional layer models of the object undergoingfabrication to determine whether the areas of the powdered mediumcorresponding to said layer are being heated to appropriate temperaturesto effect fusing of the medium. Similarly, adjacent areas of thepowdered medium that do not form part of the cross-sectional layer ofthe object may be evaluated to ensure they are not being heated tomelting points of the powdered medium so as to ensure those areas arenot fused. Likewise, background heating can be evaluated and adjusted,if necessary, in accordance with images provided by imaging device 26.

In the above-described process, where a laser is used to heat thoseportions of a layer of the powdered medium to a temperature below itsmelting point, that laser and the laser used to form the spatial lightmodulated laser beam may be created from a common laser beam using abeam splitter (not shown in this view).

In the above-described process, spatial light modulation of the laser toform the spatial light modulated laser beam may be accomplished bygenerating a holographic image of a respective cross-sectional layer ofthe object on a liquid crystal on silicon (LCoS) panel, directing anon-spatial light modulated portion of the laser beam to be incident onthe holographic image, and using a reflected portion of the laser beamfrom the holographic image of the first cross-sectional layer of theobject on the LCoS panel as the spatial light modulated first laserbeam. In such instances, the holographic image of the cross-sectionallayer of the object on the LCoS panel maybe corrected for 0th orderdiffraction effects and/or other image effects in the spatial lightmodulated first laser beam at an image plane. In alternativeembodiments, spatial light modulation of the laser may be effectedthrough use of a digital micromirror device (DMD), which may becontrolled to produce a representation of the cross-sectional layer ofthe object to be fabricated. DMDs are commonly used in digital lightprojectors and typically include many microscopic mirrors arranged in anarray. The mirrors, which correspond to pixels in an image, can beindividually rotated to an “on” or “off” position, thus allowing forselected pixels of an incident laser be to be reflected or not. DMDsthus provide amplitude spatial light modulation of the incident laserbeam. Although in the remaining discussion the use of LCoS devices ishighlighted, it should be appreciated that this discussion appliesequally to the use of DMD devices in place of, or in combination with,LCoS devices.

FIG. 2 illustrates one embodiment of an optical arrangement which may beused in the apparatus discussed above in connection with FIGS. 1A-1C. Inthis example, the laser source 18 includes a laser 30 optically coupledwith a fiber collimator 32. The fiber collimator provides a collimatedlight beam 34 to a polarizing beam splitter 36, such that a portion 38of the incident beam is provided at the output of the polarizing beamsplitter. This polarized beam (e.g., linearly polarized beam) isprovided as an input to beam expander 40, which expands the beam widthof the polarized beam to produce a collimated, polarized (e.g. linearlypolarized), and expanded beam 42. Note that if the laser beam fromcollimator 32 is already polarized (e.g., linearly polarized),polarizing beam splitter 36 may be omitted from the optical arrangement.

The collimated, polarized, and expanded beam 42 is made incident upon anLCoS panel 44. The LCoS panel 44 is under computer control 50 so as tospatially light modulate the incident laser beam 42 to form a spatiallight modulated laser beam 46. Computer-based controller 50 generates aholographic image of a respective cross-sectional layer of the object tobe fabricated and displays same via LCoS panel 44, thereby causing aportion of the non-spatial light modulated incident laser beam 42 to bereflected from the LCoS panel 44 as the spatial light modulated laserbeam 46. In some embodiments, the holographic image of thecross-sectional layer of the object on the LCoS panel 44 may becorrected for 0th order diffraction effects (and/or other image effects)in the spatial light modulated laser beam 46 at an image plane on powderbed 12. The spatially light modulated laser beam 46 is directed byprojecting optics 48 (e.g., a microspot focusing objective) to the workarea 16 of powder bed 12. In some embodiments, for example as shown inFIG. 4, a collimator 52 that is optically coupled with laser 30 may beused as a beam expander to provide a collimated and expanded light beamto beam splitter 36 and LCoS panel 44.

Although not shown in the illustration, in some embodiments a portion ofthe laser beam not directed to LCoS panel 44 by beam splitter 36 may beprovided to a similar optical arrangement to direct a second laser beam(appropriately spatially light modulated by a second LCoS panel, DMD, orother arrangement) to the same work area on the powder bed. This wouldhave the effect of increasing (perhaps doubling) the light powerdelivered to the work area.

Alternatively, or in addition, a portion of the laser beam not directedto LCoS panel 44 by beam splitter 36 may be provided to a second imagingarrangement (as shown in FIG. 6) to fabricate contours around the crosssection being fused, thereby increasing the resolution of this imagedcross section. An appropriate optical arrangement for such a system mayinclude a mirror galvanometer, as are commonly used in SLS systems, orother directing system.

Also, in still further embodiments, a portion of the laser beam notdirected to LCoS panel 44 by beam splitter 36 may be directed throughappropriate optics (which may include a second spatial light modulatoror DMD) to provide a modified beam with desired energy density acrossits beam width that is subsequently used to provide background heatingof the work area, e.g., in place of or in addition to infra-red heatersto heat the powdered medium in the work space to a temperature justbelow its melting point. For example, the infra-red heaters may be usedto heat the powdered medium to a first temperature below the meltingpoint of the powdered medium, and the portion of laser beam not directedto LCoS panel 44 by beam splitter 36 may be used to heat the powderedmedium to a second temperature, the second temperature being higher thanthe first temperature but still below the melting point of the powderedmedium. Temperatures of the powdered medium may be evaluated at one ormore points using the imaging devices and/or other temperature sensors.

FIGS. 3A-3C illustrate various examples of projecting optics (48) foruse in connection with the imaging systems shown in FIG. 2, FIG. 4,and/or FIG. 6, the latter of which is discussed further below. FIG. 3Aillustrates projecting optics 140 with focusing lens 112 realizingFourier transformation and a focusing lens 113 realizing projection.FIG. 3B illustrates projecting optics 240 with a focusing lens 122realizing Fourier transformation and an objective 123 realizingprojection. FIG. 3C illustrates projecting optics 340 with a focusinglens 132 realizing Fourier transformation and a negative lens 133realizing projection.

FIG. 5 illustrates yet another embodiment of an optical arrangementwhich may be used in the apparatus discussed above in connection withFIGS. 1A-1C. In this example, the laser light is collimated and apolarizer 36 and divergent lens 41 realize projection of the laser lightonto LCoS panel 44. In this embodiment, no additional projecting opticsare used to convey the spatially light modulated laser beam 46 to thework area 16 of powder bed 12 as the laser light is already diverging.Also, if desired the polarizer 36 may not provide a beam splittingfunction. Instead, infra-red heaters may be employed as there is nosecondary beam to provide heating of the powdered material in the powderbed. Such heaters may also be used in addition to or in place ofsecondary laser beams in other embodiments of the invention.

FIG. 6 illustrates still another embodiment of an optical arrangementwhich may be used in the apparatus discussed above in connection withFIGS. 1A-1C. In this example, a mirror galvanometer (or “galvo”) unit54, e.g., composed of one or more tiltable mirror arrangements 56, 58,is used to direct secondary laser beam 60 towards work area 16 of powderbed 12. Additional optical elements, such as mirror 62 and projectingoptics 64 may also be included in the optical path of the secondarybeam. Secondary laser beam 60 may provide heating of the work area or,as illustrated, may be used to provide contouring of high definitionedges of the cross-sectional layer under fabrication.

The LCoS display used to produce a spatially light modulated beam in theabove-described embodiments is an example of a two-dimensional, phasespatial light modulator (phase SLM). It includes a number ofindividually addressable pixels, which, depending on their state (e.g.,up to 256 levels of darkness) can reshape the beam across its entireplane so that it is fully utilized in the shape of the desiredcross-section. That is, the LCoS display 44 modulates the incident lightbeam with a liquid crystal layer so that the phase of the light isaffected, preferably without any change to the light intensity andpolarization. The phase modulation is achieved with high linearity bycontrolling the liquid crystal state for each pixel (e.g., by applyingor not applying a voltage between electrodes) using, for example, acontrol signal from controller 50. One example of a phase LCoS SLMsuitable for use in systems configured in accordance with embodiments ofthe present invention is the Exulus® HD1 Spatial Light Modulatoravailable from Thorlabs of Newton, N.J. In other embodiments, anamplitude SLM may be used. Amplitude SLMs change the intensity ofincident light, rather than its phase, according to pixel state.

In each instance, phase SLM and amplitude SLM, a computer-generatedhologram is used to create a pattern on the SLM panel. The pattern is acollection of pixels in different states that is resolved to an opticalimage in the far field by reflection of the incident light beam. FIG. 7shows an example of an original image 66 that is converted to acomputer-generated hologram through the use of an appropriatelyprogrammed computer system (i.e., holographic image generator) 68. Theholographic image 70 displayed on the LCoS display reflects an incidentlaser beam as discussed above, and the resulting spatially lightmodulated beam is projected onto the work area of the powder bed so asto produce a representation 72 of the original image in a focal plane atthe work area. The image includes bright spots (areas where laser lightis incident on the powdered media present in the powder bed) and darkspots (areas where no laser light is incident). The powdered medium isfused by the laser light where it is present and remains unfused wherethe laser light is not present. In this way, a cross section of theobject under fabrication is created, an entire cross section at a time.This latter point is important. Unlike conventional SLS apparatus, inwhich the laser is scanned over the work area to trace out the shape ofthe cross section being fabricated, in the present invention an image ofan entire cross section of the object under fabrication is projectedonto the powdered medium in the powder bed, and the entire crosssectional layer is fabricated at one time through fusion of the powderedmedium in places corresponding to areas where laser light (i.e., thespatially light modulated laser beam reflected from the LCoS display) isincident.

As shown in FIG. 7, the projected image 72 has a distinct bright spot inits center. This is an example of a 0^(th) order diffraction effect andis generally an undesirable artifact in the projected image. Such imagedefects can be eliminated by adjusting the computer-generated hologrampattern displayed on the LCoS display, for example by adding Fresnellens focusing effect to that pattern. These focusing effects can beadded through the use appropriate software control of the holographicimage generator 68. An example of a computer software package forgenerating holographic images of the kind contemplated for use inaccordance with the present invention is the Exulus SLM software forpersonal computer control of the Exulus SLM, available from Thorlabs.

In the above-described embodiments, reference is made to variouscontrollers 28, 50. These controllers are examples of processor-basedcontrol devices that operate according to program instructions. In otherwords, they are specially programmed computer systems. As such, itshould be recognized that the operations referred to herein are machineoperations and various embodiments of the present invention may beimplemented with the aid of processor-implemented processes or methods(a.k.a. programs or routines) that may be rendered in anyprocessor-executable language. Such processor-implemented processes ormethods may be stored (in its processor-executable language form) in aprocessor-readable storage medium, such as, but not limited to, any typeof disk-based memory, solid state memory, or other media, which mediamay be readable only or read/writable. A controller suitable for use inconnection with the present invention will, generally, include one ormore busses or other communication mechanisms for communicatinginformation, and one or more processors coupled with the bus(ses) forprocessing information. The controllers also will include a main memory,such as a random access memory (RAM) or other dynamic storage device,coupled to the bus(ses) for storing information and instructions to beexecuted by processor(s), and for storing temporary variables or otherintermediate information during execution of instructions to be executedby processor(s). The controllers further will include one or more staticstorage devices, e.g., a read only memory (ROM), coupled to the bus(ses)for storing static information and instructions for the processor(s). Along term storage device, such as a hard disk or flash drive, may alsobe included and coupled to the bus(ses) for storing information andinstructions. Optionally, human interface devices such as a display,keyboard (of other input device), and/or mouse (or other cursor controldevice) may also be included (via coupling to the bus(ses) forcommunicating information and command selections to the processor(s).Likewise, the controllers may include communication interfaces toprovide two-way data communication with sensors, such as imagingdevices, actuators, such as lasers, optical components, spatial lightmodulators, and/or remote computer systems. These communicationinterfaces may allow for conventional, wired and/or wireless, computernetwork communications and/or communications via proprietarycommunication protocols.

Using the apparatus described above, in one embodiment of the inventiona three-dimensional article may be manufactured by laser fusion. Forexample, a first layer of a powdered medium may be distributed over apowder bed of the apparatus and heated. The heating may be effected byinfra-red heaters, and/or a laser. Where a laser is employed for thispurpose, the laser may be the same laser used for producing the crosssection of the object under fabrication, in which case an emitted laserbeam may be split and a portion thereof (appropriately modified to havea desired energy distribution over its beam width) directed on thepowdered medium in a working area of the powder bed to heat the powderedmedium to a temperature near, but not over, its melting point. In somecases, where a combination of heating means is used, one or moreinfra-red heaters may be used to heat the powdered medium to a firsttemperature, below the melting point of the powdered medium, and thelaser used to provide additional energy to heat the powdered medium to asecond, higher temperature that is still below its melting point. Thismay allow for more precise control over the ambient heating of thepowdered medium in the powder bed and the temperature of said medium maybe monitored using infra-red imaging devices so as to allow for controlof the heat energy being applied thereto.

An image of a cross-sectional layer of the object being fabricated isprojected on at least a portion of the top layer of the powdered mediumdistributed over the powder bed. In accordance with the presentinvention, this image of the cross-sectional layer is created by spatiallight modulation of a laser beam, e.g., in the fashion discussed above,and projection of a resulting spatial light modulated laser beam throughan optical path onto the top layer of the powdered medium in the powderbed. The spatial light modulated laser beam has sufficient energy tofuse that portion of the powdered medium in locations in the powder bedcorresponding to the image of the cross-sectional layer of the object.In this way, an integral layer of the powdered medium having a shapecorresponding to the image of the cross-sectional layer of the object isformed.

The same imaging devices used to monitor the heating of the powderedmedium in the powder bed can be used to monitor the fusing thereof and,based on the imaging, the power of the beam over its cross section canbe controlled automatically, for example by varying the gray level ofthe image of the cross section to be fabricated. To better understandthis operation, consider the example shown in FIG. 8. An image to beprojected onto the powdered medium in the powder bed is shown at 74.This image represents a cross-section of an object to be fabricated andis in this instance in the shape of a plus sign (“+”). The image of thecross section of the object is converted to a computer-generatedhologram 76 through the use of an appropriately programmed computersystem, as described above. The holographic image 76 is displayed on theLCoS display to produce a spatially light modulated beam that isprojected onto the work area of the powder bed. The result is arepresentation 78 of the original image in a focal plane at the workarea of the powder bed.

As shown in this example, the projection of the original image isuneven, with some areas of the image being brighter than others. If leftunchecked, this may lead to some areas of the powdered medium beingimproperly fused, while others are overmelted. Accordingly, one or moreinfra-red images 80 of the projection of the original image are capturedand analyzed 82 so that the gray levels of the original image of thecross section to be fabricated are adjusted 84. In the adjusted image84, the gray levels of the lower portion of the plus sign are darkenedin their interiors as compared to the gray level of the remainingportion of the plus sign. The adjusted image 84 is converted to acomputer-generated hologram 86, and a resulting representation 88 of theadjusted image is projected onto the focal plane at the work area of thepowder bed. As illustrated, the brightness of this representation 88 ofthe adjusted image is more even over its entirety than the previousrepresentation 78 of the original image 74, and will therefore providemore even distribution of the laser energy over the powdered medium tobe fused.

The analysis of the infra-red images may be performed visually, e.g., byan operator of the system, or may be performed robotically using one ormore machine vision analysis techniques. Machine vision analyses mayinvolve comparisons of light intensities (e.g., representative of powerdistribution or temperature distribution) over various portions of animage that represent places where the powdered medium is to be fused andplaces where no fusion is to occur. For example, thresholds may beestablished which represent maximum light intensities that should not beexceeded in areas where no fusion of the powdered medium is to occur.Any measured light intensities over that threshold would then beassociated with areas of the image which require gray level adjustmentso as to alter the intensity of the laser light at the work area of thepowder bed. Similarly, color or gray level analysis of areas wherefusion of the powdered medium is to occur could be analyzed so as toensure the light intensity is sufficiently high so as to ensure fusionof the powdered medium, but no so high as to cause vaporization thereof.

The example shown in FIG. 8 concerns an original image 74 having onlydark or light features (a binary image), but the same process may beapplied to an original image 90 having various gray levels, as shown inFIG. 9. In this example, the adjustments made to the original imagebased on the infra-red image analysis result in changes to the graylevels across the entire image, as shown at 92. These adjustmentsprovide for uniform light intensity across an entire cross section, asshown at 94.

Upon completion of the fusing of the cross-sectional area of the object,a new top layer of the powdered medium is distributed over the nowcompleted cross section (and any unfused portion of the powderedmedium), and the application of the spatially light modulated laser beamis repeated for an image corresponding to a new cross-sectional layer ofthe object being fabricated. This forms a new integral layer of thepowdered medium having a shape corresponding to the image of this newcross-sectional layer of the object. This process is repeated for aslong as desired, so as to ultimately form the three-dimensional article.As mentioned above, during this fabrication process the quality andaccuracy of reproduction of the respective projections of the images ofthe cross-sectional layers of the object at the powder bed may becontrolled by monitoring these projections with one or more imagingdevices and controlling the power of the spatially modulated laser beamincident on the powdered medium over the beam's cross section. Inaddition, times of applying the respective projections of the originalimages on respective layers of the powdered medium may also becontrolled in accordance with the monitored image analysis.

Spatial light modulation of the laser beam to form the projections ofimages on the powdered medium in the powder bed may be phase spatiallight modulation of an incident laser beam or, in other embodiments,amplitude modulation of an incident laser beam. In either case, spatiallight modulation of the incident laser beam may be effected bygenerating a holographic representation of an image of a cross-sectionallayer of the object on an LCoS display, directing a non-spatial lightmodulated laser beam to be incident on the LCoS display, and using areflected portion of the laser beam from the LCoS display as the spatiallight modulated laser beam. Preferably, the holographic representationof the image on the LCoS display is corrected for 0th order diffractioneffects and/or other optical aberrations in the spatial light modulatedlaser beam at its image plane.

In one embodiment of the invention, a work space of approximately 300 mmsquare (i.e., a target area of 90,000 mm²) is provided. To print a layerin under 10 secs, an estimated optical power of approximately 700 W maybe required. When the workspace is only 40 mm (1600 mm²), an estimatedoptical power of only approximately 13 W is expected to be needed.

The foregoing embodiments are discussed in connection with a reflectiveLCoS panel, but other embodiments may employ a transmissive LCoS panel.In such instances, images are provided by transmission of a laser beamthrough a holographic representation of an image displayed in the paneland onto an image plane at the powder bed.

As described above, the image of the cross section to be fabricated maybe modified (e.g., by gray level adjustments) prior to generation of theholographic representation of the image so that the resulting spatiallylight modulated laser beam has a non-uniform intensity across its crosssection. This allows some pixels at the image plane (i.e., areas of thepowdered medium at the powder bed) to be more or less intense thanothers. Thus, the projection of the image can be controlled so that, forexample, edges of the cross section of the object being fabricated areimaged with light energy of a different intensity than other portions ofthe cross section in order to achieve better (more precise) edgequality. Similarly, areas of the cross section of the object beingfabricated may be imaged with less intense light energy than otherportions of the cross section in order to reduce the accumulation ofheat in the interior or other areas of the article under fabrication.This may be accomplished, for example, by graying out areas of theholographic representation of the image of the cross-sectional layer ofthe three-dimensional article so that corresponding portions of thespatially light modulated laser beam incident on the powdered medium areless intense than other portions of the beam. The gray level adjustmentsto areas of the holographic representation of the image of thecross-sectional layer of the three-dimensional article may be createdresponsively to image analysis of a projection of the original image ofthe cross-sectional layer on the powdered medium in the powder bed, forexample as provided by one or more of the above-mentioned imagingdevices.

Thus, methods and apparatus as facilitate concurrent formation ofcross-sectional layers of an object have been described. In variousembodiments of the invention, a three-dimensional article may be formedby powder bed fusion of individual layers in which respective ones oflayers of a powdered medium are distributed over a powder bed; at leastportions of the respective layers of the powdered medium distributedover the powder bed are heated to a temperature below a melting point ofthe powdered medium; and images of respective cross-sectional layers ofthe three-dimensional article are projected on the respective layers ofthe powdered medium distributed over the powder bed. The projectedimages of the respective cross-sectional layers are created by spatiallight modulation of a first laser beam and projection of a resultingspatial light modulated laser beam through an optical path onto therespective layers of the powdered medium. The spatial light modulatedlaser beam preferably has sufficient energy to fuse that portion of thepowdered medium in locations in the powder bed corresponding to athen-projected image of a respective cross-sectional layer of thethree-dimensional article so as to form an integral layer of thepowdered medium having a shape corresponding to that image of therespective cross-sectional layer of the three-dimensional article. Afterforming such an integral layer, a next one of the respective layers ofthe powdered medium are distributed over a powder bed (e.g., using aroller process), and the procedure repeats for that next layer and animage of the associated next cross-sectional layer of thethree-dimensional article. Through this iterative process, thethree-dimensional article is fashioned.

In the above-described embodiments, for each image of a respectivecross-sectional layer of the three-dimensional article, spatial lightmodulation of the first laser beam may include generating arepresentation of the image of the respective cross-sectional layer ofthe three-dimensional article on a liquid crystal on silicon panel orother modulation device (e.g., a DMD), directing a non-spatial lightmodulated portion of the first laser beam to be incident on therepresentation of the image of the respective cross-sectional layer, andusing a reflected or transmitted portion of the first laser beam fromthe representation of the image of the respective cross-sectional layeron the liquid crystal on silicon panel as the spatial light modulatedlaser beam. Qualities of respective projected images of thecross-sectional layers of the three-dimensional article at the powderbed may be controlled by monitoring the respective projected images withone or more imaging devices (e.g., infra-red imaging devices, CCDcameras, etc.) and adjusting characteristics of the correspondingrespective images of the cross-sectional layers of the three-dimensionalarticle in accordance therewith. For example, gray levels of portions ofthe corresponding respective images may be adjusted to provide even orother desired illumination of the powdered medium in the powder bed bythe spatial light modulated laser beam. The spatial light modulationeffected by the spatial light modulation device may be phase spatiallight modulation or amplitude spatial light modulation, and theholographic representation of the image of the cross-sectional layersmay be corrected for 0th order diffraction effects in the spatial lightmodulated laser beam at an image plane.

In the above-described embodiments, the powdered medium distributed overthe powder bed may be heated to a temperature below the melting point ofthe powdered medium by one or more of: a second laser beam directedtowards the powder bed through a second optical path (which secondoptical path may include a spatial light modulation device such as aliquid crystal on silicon device or a digital micromirror device, ordiffractive optics), and one or more infra-red heaters. Where a secondlaser beam is used for heating purposes, the first laser beam and thesecond laser beam may be created from a common laser beam using apolarizing beam splitter. Heating may be formed so that the infra-redheaters heat the powdered medium to a first temperature, and the laserheats the powdered medium to a second temperature.

In some embodiments of the invention, respective second images of eachrespective cross-sectional layer of the three-dimensional article may beprojected to respective layers of the powdered medium (either theirrespective entireties or only the edges thereof) distributed over thepowder bed, wherein the respective second projected images of therespective cross-sectional layers are created by spatial lightmodulation of a second laser beam and projection of a resulting secondspatial light modulated laser beam through a second optical path (whichmay include a mirror galvanometer arrangement or other directingarrangement) onto said respective layers of the powdered medium. Such anembodiment facilitates contouring of the edges of the article underfabrication. In such instances, the first and second laser beams may beformed from a common laser beam, for example using a polarizing beamsplitter. In some cases, a mirror galvanometer arrangement is includedin the second optical path, and respective second images of eachrespective cross-sectional layer of the three-dimensional article areprojected on the respective layers of the powdered medium distributedover the powder bed.

As with the above-described embodiments, for each image of a respectivecross-sectional layer of the three-dimensional article, spatial lightmodulation of the first laser beam may be effected using a holographicrepresentation of the image of the respective cross-sectional layer ofthe three-dimensional article on a liquid crystal on silicon panel toprovide a phase or amplitude modulated spatial light modulated laserbeam from a non-spatial light modulated portion of the first laser beamincident on the liquid crystal on silicon panel. The holographicrepresentation of the image of the respective cross-sectional layer ofthe three-dimensional article may be created from a modified version ofthe image of the respective cross-sectional layer of thethree-dimensional article, where the modified version of the image ofthe respective cross-sectional layer has gray levels thereof adjustedfrom those associated with an original image of the respectivecross-sectional layer of the three-dimensional article according toimage analysis of the original image of the respective cross-sectionallayer, either before or after projection on the powdered medium in thepowder bed. Similarly, spatial light modulation of the first laser beammay be effected using a liquid crystal on silicon panel, a DMD, ordiffractive optics.

EXAMPLE EMBODIMENTS

A A method for forming a three-dimensional article by powder bed fusion,comprising the steps of: (a) distributing a first layer of a powderedmedium over a powder bed; (b) heating the first layer of the powderedmedium distributed over said powder bed to a temperature below a meltingpoint of the powdered medium; (c) projecting an image of a firstcross-sectional layer of the three-dimensional article on the firstlayer of the powdered medium distributed over said powder bed, wherein aprojected image of the first cross-sectional layer is created by spatiallight modulation of a first laser beam and projection of a resultingspatial light modulated laser beam through a first optical path onto thefirst layer of the powdered medium, said spatial light modulated laserbeam having sufficient energy to fuse that portion of the powderedmedium in locations in the powder bed corresponding to said image of thefirst cross-sectional layer of the three-dimensional article so as toform a fused portion of the first layer of said powdered medium having ashape corresponding to said image of the first cross-sectional layer ofthe three-dimensional article, (d) distributing a second layer of thepowdered medium over the first layer of the powdered medium, andrepeating steps (b) and (c) the second layer and an image of a secondcross-sectional layer of the three-dimensional article, respectively, soas to form a fused portion of the second layer of said powdered mediumhaving a shape corresponding to said image of the second cross-sectionallayer of the three-dimensional article; and (e) further repeating step(d) for additional layers of the powdered medium, each individual one ofsaid additional layers being distributed over an immediately precedinglayer of the powdered medium, additional fused portions of respectiveones of the additional layers, and additional images of respectiveadditional cross-sectional layer of the three-dimensional article, so asto form the three-dimensional article.

B. The method of embodiment A, wherein, for each image of a respectivecross-sectional layer of the three-dimensional article, spatial lightmodulation of the first laser beam comprises generating a representationof the image of the respective cross-sectional layer of thethree-dimensional article on a spatial light modulator, directing anon-spatial light modulated portion of the first laser beam to beincident on the holographic representation of the image of therespective cross-sectional layer, and using a reflected portion of thefirst laser beam from the representation of the image of the respectivecross-sectional layer on the spatial light modulator as the spatiallight modulated laser beam, wherein the spatial light modulatorcomprises one of a liquid crystal on silicon panel, or a digitalmicromirror device.

C. The method of embodiment B, wherein qualities of respective projectedimages of the cross-sectional layers of the three-dimensional article atthe powder bed are controlled by monitoring measured power ortemperatures distributions on the respective projected images during theprojecting of the image and fusing of the powdered medium with one ormore imaging devices, and adjusting gray levels of correspondingrespective images of the cross-sectional layers of the three-dimensionalarticle or times of fusing of the powdered medium to ensure said powerintensities are sufficiently high enough to fuse the powdered mediumwithout overmelting it.

D. The method of embodiment B, wherein light intensities of portions ofrespective projected images of the cross-sectional layers of thethree-dimensional article at the powder bed are controlled by monitoringrespective projected images during the projecting of the image andfusing of the powdered medium with one or more imaging devices, andadjusting gray level portions of corresponding respective images of thecross-sectional layers of the three-dimensional article.

E. The method of embodiment B, wherein a time of fusing the powderedmedium is controlled by monitoring feedback images of the powderedmedium during the projecting of the image and fusing of the powderedmedium produced by one or more imaging devices.

F. The method of embodiment B, wherein for one or more of the layers ofthe powdered medium, the respective image of the respectivecross-sectional layer of the three-dimensional article is graylevel-adjusted from an original version thereof according to imageanalysis of a projection of the respective image of the respectivecross-sectional layer of the three-dimensional article on the powderedmedium in the powder bed before the projection of the image and fusionof the powdered medium.

G. The method of embodiment B, further comprising projecting respectivesecond images of each respective cross-sectional layer of thethree-dimensional article on the respective layers of the powderedmedium distributed over said powder bed, wherein respective secondprojected images of the respective cross-sectional layers are created byspatial light modulation of a second laser beam and projection of aresulting second spatial light modulated laser beam through a secondoptical path onto said respective layers of the powdered medium, whereinthe spatial light modulation is provided by a liquid crystal on siliconpanel, digital micromirror device, or diffractive optics.

H. The method of embodiment G, wherein spatial light modulation of thesecond laser beam and projection of the resulting second spatial lightmodulated laser beam through the second optical path includes directingthe second laser beam with a galvo mirror.

I. The method of embodiment H, wherein spatial light modulation of thesecond laser beam comprises spatial light modulation using one of aliquid crystal on silicon panel, a digital micromirror device, ordiffractive optics.

J. The method of embodiment A, wherein the heating of the respectivelayers of the powdered medium distributed over said powder bed isperformed by heating the powdered medium distributed over said powderbed to a first temperature by an infra-red heating element, and heatingthe powdered medium distributed over said powder bed to a secondtemperature higher than the first temperature by directing a secondlaser beam towards the powder bed through a second optical path thatincludes a second spatial light modulator.

K. The method of embodiment B, wherein the respective representations ofthe images of the respective cross-sectional layers of thethree-dimensional article are corrected for distortions in the spatiallight modulated laser beam at an image plane.

L. The method of embodiment A, wherein, for each image of a respectivecross-sectional layer of the three-dimensional article, spatial lightmodulation of the first laser beam comprises generating a holographicrepresentation of the image of the respective cross-sectional layer ofthe three-dimensional article on a liquid crystal on silicon panel,directing a non-spatial light modulated portion of the first laser beamto be incident on the holographic representation of the image of therespective cross-sectional layer, and using a transmitted portion of thefirst laser beam from the holographic representation of the image of therespective cross-sectional layer on the liquid crystal on silicon panelas the spatial light modulated laser beam.

M. The method of embodiment I, wherein qualities of respective projectedimages of the cross-sectional layers of the three-dimensional article atthe powder bed are controlled by monitoring said respective projectedimages with one or more imaging devices and adjusting characteristics ofcorresponding respective images of the cross-sectional layers of thethree-dimensional article in accordance therewith.

N The method of embodiment J, wherein the characteristics of thecorresponding respective images of the cross-sectional layers of thethree-dimensional article that are adjusted comprise gray levels ofportions of the corresponding respective images.

O. A powder bed fusion apparatus, comprising: means for distributingrespective layers of a powdered medium over a powder bed; means forheating a current one of said respective layers of the powdered mediumdistributed over said powder bed to a temperature below a melting pointof the powdered medium; means for projecting respective images ofrespective cross-sectional layers of an object on the current one of therespective layers of the powdered medium distributed over said powderbed, a respective projected image of a respective cross-sectional layerbeing created by spatial light modulation of a first laser beam andprojection of a resulting spatial light modulated laser beam through afirst optical path onto said current one of the respective layers of thepowdered medium so as to form a fused portion current one of therespective layers of the powdered medium in locations in the powder bedhaving a shape corresponding to said respective image of the respectivecross-sectional layer of the object; a control system coupled to themeans for distributing, means for heating, and means for projecting,said control system including a processor and a memory coupled to theprocessor, the memory storing instructions, which instructions whenexecuted by the processor cause the processor to, for each respectivecross-sectional layer of the object: cause the means for distributing todistribute, one at a time, said respective layers of the powdered mediumover the powder bed, each successive respective layer of the powderedmedium being distributed over a previously distributed one of therespective layers of the powdered medium, cause the means for heating toheat the current one of the respective layers of the powdered mediumdistributed over said powder bed to a temperature below a melting pointof the powdered medium, and cause the means for projecting therespective projected image of the respective cross-sectional layer ofthe object being fabricated onto the current one of the respectivelayers of the powdered medium then-distributed over said powder bed byspatial light modulation of the first laser beam.

P. The powder bed fusion apparatus of embodiment O, wherein the meansfor projecting comprises one of a liquid crystal on silicon panel, or adigital micromirror device, and the memory stores further instructions,which further instructions when executed by the processor cause theprocessor to, for each respective cross-sectional layer of the object,generate a representation of a respective image of the respectivecross-sectional layer of the object on the liquid crystal on siliconpanel or digital micromirror device, as appropriate, direct anon-spatial light modulated portion of the first laser beam to beincident on the representation of the image of the respectivecross-sectional layer, and use a reflected portion of the first laserbeam from the representation of the image of the respectivecross-sectional layer on the liquid crystal on silicon panel or digitalmicromirror device, as appropriate, as the spatial light modulated laserbeam.

Q. The powder bed fusion apparatus of embodiment P, further comprising afeedback system including one or more imaging devices, said feedbacksystem coupled to the control system, and wherein the memory storessecond further instructions, which second further instructions whenexecuted by the processor cause the processor to monitor the respectiveimages of the cross-sectional layers of the object at the powder bedusing images from the one or more imaging devices, determine qualitiesof the respective images of the cross-sectional layers of the object,and adjust characteristics of corresponding respective images of thecross-sectional layers of the three-dimensional article in accordancetherewith.

R. The powder bed fusion apparatus of embodiment Q, wherein thecharacteristics of the corresponding respective images of thecross-sectional layers of the three-dimensional article that areadjusted comprise gray levels of portions of the correspondingrespective images.

S. The powder bed fusion apparatus of embodiment O, wherein the meansfor heating comprises one or more infra-red heating devices and a secondlaser beam directed to the powder bed through a second optical path, andwherein the memory further stores instructions which when executed bythe processor cause the processor to control the infra-red heatingelements and the second laser beam to heat the respective layers of thepowdered medium distributed over said powder bed to a temperature belowa melting point of the powdered medium in stages, using the infra-redheating devices to heat the respective layers of the powdered medium toa first temperature and the second laser beam to heat the respectivelayers of the powdered medium to a second temperature, above the firsttemperature, where both the first temperature and the second temperatureare below the melting point of the powdered medium.

T. The powder bed fusion apparatus of embodiment S, wherein the secondoptical path provides spatial light modulation of the second laser beam.

U. The powder bed fusion apparatus of embodiment T wherein the spatiallight modulation of the second laser beam is effected by one of a secondliquid crystal on silicon panel or second digital micromirror device.

V. The powder bed fusion apparatus of embodiment S, further comprising apolarizing beam splitter in an optical path between a laser source andthe means for projecting, wherein the first laser beam and a secondlaser beam are created from a common laser beam incident on thepolarizing beam splitter.

W. The powder bed fusion apparatus of embodiment O, wherein the meansfor projecting includes a liquid crystal on silicon panel and whereinthe memory stores further instructions, which further instructions whenexecuted by the processor cause the processor to, for each respectivecross-sectional layer of the object, generate a holographicrepresentation of a respective image of the respective cross-sectionallayer of the object on the liquid crystal on silicon panel, direct anon-spatial light modulated portion of the first laser beam to beincident on the holographic representation of the image of therespective cross-sectional layer, and use a transmitted portion of thefirst laser beam from the holographic representation of the image of therespective cross-sectional layer on the liquid crystal on silicon panelas the spatial light modulated laser beam.

X. The powder bed fusion apparatus of embodiment W, wherein the memorystores second further instructions, which second further instructionswhen executed by the processor cause the processor to control graylevels of portions of respective projected images of the cross-sectionallayers of the object at the powder bed by monitoring said respectiveprojected images with one or more imaging devices and adjusting the graylevels of portions of corresponding respective images of thecross-sectional layers of the three-dimensional article in accordancetherewith.

Y. The powder bed fusion apparatus of embodiment O, wherein the meansfor projecting further comprises a second optical path and the memorystores further instructions, which further instructions when executed bythe processor, cause the processor to control the means for projectingto project respective second respective images of each respectivecross-sectional layer of the object on respective layers of the powderedmedium distributed over said powder bed, wherein the second respectiveprojected images of the respective cross-sectional layers are created bydirecting a second laser beam through the second optical path, whichcomprises one or more of a liquid crystal on silicon panel, a digitalmicromirror device, diffractive optics, and a galvo mirror, andprojection of a resulting laser beam onto said respective layers of thepowdered medium.

Z. The powder bed fusion apparatus of embodiment Y, wherein the firstand second laser beams are formed from a common laser beam using apolarizing beam splitter.

AA. The powder bed fusion apparatus of embodiment OO, wherein therepresentation of the image of the respective cross-sectional layer ofthe object is created from a modified version of the image of therespective cross-sectional layer of the object, said modified version ofthe image of the respective cross-sectional layer having gray levelsthereof adjusted from those associated with an original image of therespective cross-sectional layer of the object according to imageanalysis of a projection of the original image of the respectivecross-sectional layer on the powdered medium in the powder bed.

BB. An additive manufacturing apparatus, comprising: a powder storagesystem and powder bed for containing a powder medium a first opticalpath for projecting respective images of cross-sections of an object tobe fabricated onto respective layers of the powder medium in the powderbed; and a control system coupled to and configured to operate thepowder storage system to distribute, one at a time, said respectivelayers of the powder medium over the powder bed, each successiverespective layer of the powder medium being distributed over apreviously distributed one of the respective layers of the powdermedium, coupled to and configured to operate components of the firstoptical path to project, one at a time, the respective images ofcross-sections of the object being fabricated onto a current one of therespective layers of the powder medium then-distributed over said powderbed.

CC. The additive manufacturing apparatus of embodiment BB, wherein thefirst optical path comprises a laser source optically coupled to providean incident collimated light beam to a polarizing beam splitter, suchthat a portion of the incident beam is provided at an output of thepolarizing beam splitter as a polarized beam, said polarized beam beingprovided as an input to beam expander optically coupled to thepolarizing beam splitter, said beam expander for expanding a beam widthof the polarized beam to produce an expanded beam that is made incidentupon a spatial light modulating apparatus optically coupled to the beamexpander.

DD. The additive manufacturing apparatus of embodiment CC, wherein thelaser source includes a laser optically coupled with a fiber collimator.

EE. The additive manufacturing apparatus of embodiment CC, wherein thespatial light modulating apparatus comprises one of a liquid crystal onsilicon panel or digital micromirror device.

FF. The additive manufacturing apparatus of embodiment CC, wherein thefirst optical path further comprises projecting optics optically coupledto receive a spatially light modulated laser beam from the spatial lightmodulating apparatus and to project the spatially light modulated laserbeam onto the powder bed.

GG. The additive manufacturing apparatus of embodiment FF, wherein theprojecting optics comprise one of: projecting optics with a focusinglens realizing Fourier transformation and a focusing lens realizingprojection; projecting optics with a focusing lens realizing Fouriertransformation and an objective realizing projection; and projectingoptics with a focusing lens realizing Fourier transformation and anegative lens realizing projection.

HH. The additive manufacturing apparatus of embodiment BB, wherein thefirst optical path comprises a laser source optically coupled to providean incident collimated light beam to a collimator that is opticallycoupled with the laser source and which provides a collimated andexpanded light beam to a polarizing beam splitter, such that a portionof the incident beam is provided at an output of the polarizing beamsplitter as a polarized beam, said polarized beam being made incidentupon a spatial light modulating apparatus optically coupled to thepolarizing beam splitter.

II. The additive manufacturing apparatus of embodiment HH, wherein thespatial light modulating apparatus comprises one of a liquid crystal onsilicon panel or digital micromirror device.

JJ. The additive manufacturing apparatus of embodiment HH, wherein thefirst optical path further comprises projecting optics optically coupledto receive a spatially light modulated laser beam from the spatial lightmodulating apparatus and to project the spatially light modulated laserbeam onto the powder bed.

KK. The additive manufacturing apparatus of embodiment JJ, wherein theprojecting optics comprise one of: projecting optics with a focusinglens realizing Fourier transformation and a focusing lens realizingprojection; projecting optics with a focusing lens realizing Fouriertransformation and an objective realizing projection; and projectingoptics with a focusing lens realizing Fourier transformation and anegative lens realizing projection.

LL. The additive manufacturing apparatus of embodiment BB, wherein thefirst optical path comprises a laser source optically coupled to providean incident collimated light beam to a polarizer, such that a portion ofthe incident beam is provided at an output of the polarizer as apolarized beam, said polarized beam being received as an input by adivergent lens optically coupled to the polarizer and producing anoutput diverging beam made incident upon a spatial light modulatingapparatus optically coupled to the divergent lens.

MM. The additive manufacturing apparatus of embodiment LL, wherein thespatial light modulating apparatus comprises one of a liquid crystal onsilicon panel or digital micromirror device.

NN. The additive manufacturing apparatus of embodiment BB furthercomprising a second optical path, said second optical path including agalvo unit 54 optically coupled to receive a component of the output ofthe polarizing beam splitter and direct said component as a second laserbeam onto the powder bed.

Thus, methods and apparatus for additive manufacturing, and inparticular to such methods and apparatus as facilitate formation ofentire cross-sectional layers of an object at a time, have beendescribed.

What is claimed is:
 1. A method for forming a three-dimensional articleby powder bed fusion, the method comprising: (a) distributing a firstlayer of a powdered medium over a powder bed; (b) heating the firstlayer of the powdered medium distributed over said powder bed to atemperature below a melting point of the powdered medium; (c) projectingan image of a first cross-sectional layer of the three-dimensionalarticle on the first layer of the powdered medium distributed over saidpowder bed, wherein a projected image of the first cross-sectional layeris created by spatial light modulation of a first laser beam andprojection of a resulting spatially light modulated laser beam through afirst optical path onto the first layer of the powdered medium, saidspatially light modulated laser beam having sufficient energy to fuse aportion of the powdered medium in locations in the powder bedcorresponding to said image of the first cross-sectional layer of thethree-dimensional article so as to form a fused portion of the firstlayer of said powdered medium having a shape corresponding to said imageof the first cross-sectional layer of the three-dimensional article; (d)distributing a second layer of the powdered medium over the first layerof the powdered medium, and repeating steps (b) and (c) for the secondlayer and an image of a second cross-sectional layer of thethree-dimensional article, respectively, so as to form a fused portionof the second layer of said powdered medium having a shape correspondingto said image of the second cross-sectional layer of thethree-dimensional article; and (e) further repeating step (d) foradditional layers of the powdered medium, each individual one of saidadditional layers being distributed over an immediately preceding layerof the powdered medium, additional fused portions of respective ones ofthe additional layers, and additional images of respective additionalcross-sectional layer of the three-dimensional article, so as to formthe three-dimensional article, wherein, for each image of a respectivecross-sectional layer of the three-dimensional article, spatial lightmodulation of the first laser beam comprises generating a representationof the image of the respective cross-sectional layer of thethree-dimensional article on a first liquid crystal on silicon (LCoS)panel, directing the first laser beam to be incident on therepresentation of the image of the respective cross-sectional layer, andusing a portion of the first laser beam reflected or transmitted fromthe representation of the image of the respective cross-sectional layeron the first LCoS panel as the spatially light modulated laser beam, themethod further comprising: projecting respective second images of eachrespective cross-sectional layer of the three-dimensional article on therespective layers of the powdered medium distributed over said powderbed through a second optical path onto said respective layers of thepowdered medium, wherein said projecting of the respective second imagescomprises one of: spatial light modulating a second laser beam using asecond LCoS panel, a second digital micromirror device, or diffractiveoptics, and projecting a resulting second spatially light modulatedlaser beam through the second optical path, or directing a second laserbeam with a mirror galvanometer and projecting a resulting second laserbeam through the second optical path, wherein the first and second laserbeams are formed from a common laser beam using a polarizing beamsplitter, and wherein the first LCoS panel is configured to receive thefirst laser beam from the polarizing beam splitter.
 2. The method ofclaim 1, wherein qualities of portions of respective projected images ofthe cross-sectional layers of the three-dimensional article at thepowder bed are controlled by monitoring measured power or temperaturedistribution on the respective projected images during the projecting ofthe image and fusing of the powdered medium with one or more imagingdevices, and adjusting (i) gray level portions of correspondingrespective images of the cross-sectional layers of the three-dimensionalarticle, or (ii) time of fusing the powdered medium, to ensure saidpower intensities are sufficiently high enough to fuse the powderedmedium without overmelting it.
 3. The method of claim 1, wherein for oneor more of the layers of the powdered medium, the respective image ofthe respective cross-sectional layer of the three-dimensional article isgray level-adjusted from an original version thereof according to imageanalysis of the respective image of the respective cross-sectional layerof the three-dimensional article before the projection of the respectiveimage and fusion of the powdered medium.
 4. The method of claim 1,wherein the heating of the respective layers of the powdered mediumdistributed over said powder bed is performed by heating the powderedmedium distributed over said powder bed to a first temperature by aninfra-red heating element, and heating the powdered medium distributedover said powder bed to a second temperature higher than the firsttemperature by directing the second laser beam towards the powder bedthrough the second optical path that includes a second spatial lightmodulator.